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Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

Butsky, Iryna S. and Fielding, Drummond B. and Hayward, Christopher C. and Hummels, Cameron B. and Quinn, Thomas R. and Werk, Jessica K. (2020) Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium. Astrophysical Journal, 903 (2). Art. No. 77. ISSN 1538-4357.

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Large reservoirs of cold (~10⁴ K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic-ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic-ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic-ray pressure. Cosmic-ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic-ray transport, by streaming or diffusion, redistributes cosmic-ray pressure from the cold gas to the background medium, resulting in cold gas properties that are in between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper ItemSimulation movies
Butsky, Iryna S.0000-0003-1257-5007
Fielding, Drummond B.0000-0003-3806-8548
Hayward, Christopher C.0000-0003-4073-3236
Hummels, Cameron B.0000-0002-3817-8133
Werk, Jessica K.0000-0002-0355-0134
Additional Information:© 2020 The American Astronomical Society. Received 2020 August 10; revised 2020 September 18; accepted 2020 September 21; published 2020 November 4. The authors thank Greg Bryan, Yan-Fei Jiang, Philip Hopkins, Philipp Kempski, Suoqing Ji, Adam Jermyn, Peng Oh, and the anonymous referee for stimulating discussions. I.B. was supported by the Simons Foundation through the Flatiron Institute's Pre-Doctoral Research Fellowship and by her tenure as a Blue Waters Graduate Fellow. The Blue Waters sustained-petascale computing project is supported by the National Science Foundation (grant Nos. OCI-0725070 and ACI-1238993) and the state of Illinois. We performed the bulk of our analysis using yt (Turk et al. 2011).
Funding AgencyGrant Number
Simons FoundationUNSPECIFIED
Flatiron InstituteUNSPECIFIED
State of IllinoisUNSPECIFIED
Subject Keywords:Astrophysical fluid dynamics ; Circumgalactic medium ; Cosmic rays ; Galaxy accretion ; Galaxy evolution ; Magnetohydrodynamical simulations
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Astrophysical fluid dynamics (101); Circumgalactic medium (1879); Cosmic rays (329); Galaxy accretion (575); Galaxy evolution (594); Magnetohydrodynamical simulations (1966)
Record Number:CaltechAUTHORS:20201104-145215845
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Official Citation:Iryna S. Butsky et al 2020 ApJ 903 77
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106436
Deposited By: Tony Diaz
Deposited On:05 Nov 2020 00:35
Last Modified:05 Nov 2020 00:35

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